1. Field of the Invention
The present invention is generally related to semiconductor fabrication. In particular, the present invention relates to a conductive metal film stack in a semiconductor.
2. Description of the Related Art
Electrical conductors are a fundamental part of integrated circuits. Electrical conductors can be used to connect devices and form conductive films that, in turn, can also be used as electrodes in a capacitor that is used to store charge in a memory cell. Preferably, integrated circuit capacitors feature a relatively large amount of capacitance in a small geometry to preserve space on the chip.
Conventional techniques for shrinking capacitor geometries include processing steps that increase the area of electrodes used as capacitor plates and include the use of high-k materials for the dielectric.
Desirable characteristics for a conductive film include relatively good conductivity and relatively good resistance to the diffusion of oxygen. Relatively good conductivity allows a relatively low contact string resistance to a device. Relatively good resistance to the diffusion of oxygen provides protection to sensitive materials, particularly where the fabrication includes formation of high-k dielectric materials. Typical high-k dielectric materials, such as barium strontium titanate (BST) (BaxSr1-xTiO3), are rich in oxygen. A highly oxidizing environment is typically required during formation of such high-k dielectric materials. Additionally, oxygen can diffuse from the high-k dielectric material after deposition, leaving conductive leakage paths in the dielectric as well as oxidizing neighboring materials.
In the past, various metals and alloys have been deposited as electrical conductors. For example, platinum (Pt) is one metal that is frequently deposited. However, films formed from platinum provide a relatively poor barrier to oxygen and allow underlying layers to oxidize. For example, if the platinum is deposited directly on silicon (Si), the diffused oxygen can result in the conversion of silicon (Si) to silicon dioxide (SiO2), which in turn results in an increase in the contact string resistance to the affected device. In another example, where platinum is deposited on a layer of tantalum that is used as an adhesion layer, the diffused oxygen can convert the tantalum (Ta) to an oxide of tantalum, thereby reducing the conductivity of the original tantalum adhesion layer.
Embodiments of the present invention provide an electrically conductive film stack for semiconductors and methods and apparatus for providing the same. The film stack can be used either as a conductor itself or in conjunction with another conductor. Advantageously, the film stack can form an oxygen barrier that substantially prevents the loss of oxygen from high-k (high dielectric constant) dielectric material and prevents oxidation of neighboring materials, such as silicon (Si). The film stack can be deposited on a three-dimensional structure, such as a wall of a high-k metal-insulator-metal (MIM) capacitor, to form an electrode for the capacitor. The electrode formed by the film stack includes graining, which enhances the surface area of the electrode and thereby increases the capacitance of the capacitor.
One process according to an embodiment of the present invention deposits an electrode for a metal-insulator-metal capacitor. The electrode serves as both an electrically conductive plate for the capacitor as well as an oxygen barrier, thereby protecting neighboring materials. The electrode is deposited in multiple layers. A first layer of the multiple layers is a layer of platinum-rhodium (Pt-Rh) alloy deposited in a metal organic chemical vapor deposition (MOCVD) process in the presence of an oxidizer. A second layer of the platinum-rhodium alloy is deposited on the first layer in the presence of a reducer. A third layer of the platinum-rhodium alloy is deposited on the second layer, again in the presence of an oxidizer.
Another process according to an embodiment of the present invention deposits a metallic film stack on a substrate. A first film is deposited with a reducer and a second film of a similar composition is deposited on the first film with an oxidizer.
In another process according to an embodiment of the present invention, the process deposits a metallic film stack by alternating between depositing a layer in the presence of a reducing agent, and depositing a layer in the presence of an oxidizing agent, while using the same metal source gas(es) during both steps. The depositing of layers can be repeated to build up a thickness of the film stack.
One system according to an embodiment of the present invention provides a MOCVD system that is adapted to alternately introduce an oxidizing reactant gas and a reducing reactant gas. A plurality of vaporizers can prepare organometallic platinum and organometallic rhodium for introduction to the deposition chamber via a relatively inert gas, such as argon (Ar) or helium (He).
One embodiment of the present invention includes a platinum-rhodium alloy capacitor plate that is deposited on a substrate. The capacitor plate includes a rough texture to enhance the capacitance of the capacitor cell. Preferably, one surface of the capacitor plate exhibits hemispherical grains with an average size of about 50 Angstroms (xc3x85) to about 1000 xc3x85, and more preferably of about 100 xc3x85 to about 500 xc3x85. It will be understood by one of ordinary skill in the art that an optimal area enhancement through the HSG Pt-Rh will vary depending on the geometry of the capacitor. Preferably, the platinum-rhodium alloy is about 5% to 50% rhodium by atomic ratio. More preferably, the platinum-rhodium alloy is about 10% to 40% rhodium by atomic ratio. Additionally, the capacitor plate is preferably from about 50 xc3x85 to about 1000 xc3x85 thick. More preferably, the capacitor plate is about 100 xc3x85 to about 600 xc3x85 thick. Advantageously, the capacitor plate provides a barrier to oxygen, thereby substantially preventing the oxidation of neighboring materials and allowing the use of high-k dielectrics.
Another embodiment according to the present invention includes a metal-insulator-metal integrated circuit capacitor. In the metal-insulator-metal capacitor, at least one of the plates or electrodes is made from a platinum-rhodium alloy. The platinum-rhodium alloy plate advantageously includes a grainy surface that faces the dielectric, and further advantageously provides an oxygen barrier for the dielectric. Preferably, the dielectric is a relatively high-k material with a relative permittivity of at least 5, or more preferably at least 10. In one embodiment, the dielectric material selected is from tantalum oxide (Ta2O5) and barium strontium titanate (BST) (BaxSr1-xTiO3).